The quantum dot solar cell concept is proposed as a scheme for increased solar cell efficiency. A theoretical model is presented for a practical p–i–n quantum dot solar cell, based on the self-organized InAs/GaAs system. The advantages of using the quantum dot in the active region for photon absorption in the long-wavelength part of the spectrum, leading to cell efficiency, is discussed.

We have investigated the influence of post-deposition annealing on the optical and electrical properties of c-axis oriented zinc oxide films prepared on sapphire substrates by electron beam evaporation. The ZnO films as-deposited and annealed in air were colourless and transparent in visible range and had sharp ultraviolet absorption edges. It is found that the optical bandgap energy of the films lies in the range of ∼3.27 to ∼3.30 eV depending on the annealing regime. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states extending into the bandgap was obtained and a value of 44 meV can be achieved by annealing in air.

We have fabricated two-dimensional in-plane p-n junctions from a p-doped pseudomorphic GaAs∕In0.1Ga0.9As∕Al0.35Ga0.65As heterostructure employing Si compensation doping by focused ion beam implantation. The current–voltage characteristics at room temperature showed good rectifying behavior, and the current in the reverse direction was below 1nA for voltages up to 10V. The depletion width was measured by optical beam-induced current, and a linear dependence on the reverse bias was found which is in contrast to the square root dependence observed in three-dimensional junctions. The results agree well with theoretical predictions.

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